10 Best IT Consulting Firms for Legacy Modernization in 2026

The real question is which IT consulting partner can solve it without disrupting the operations that run on those systems every day.

The best IT consulting firms for legacy modernization in 2026 are not just cloud migration vendors. They are strategic partners who assess your entire application portfolio, identify which systems need rearchitecting versus rehosting, design a sequenced modernization roadmap, and deliver it in phases that keep the business running while the new foundation is being built.

Legacy infrastructure absorbs 60 to 80 percent of enterprise IT spending across most large organizations. The best legacy modernization services now use AI and generative AI to cut delivery timelines by 70 to 80 percent compared to manual approaches. The firms below are delivering exactly that in 2026.

Before evaluating any firm, ask four questions. Does the firm begin with an assessment or a technology recommendation? Do they have verifiable experience in your specific industry and compliance environment? Can they show reference cases with measurable business outcomes rather than just successful deployments? And do they have the cloud modernization consulting depth to design the architecture the modernized system will run on, not just move the existing one?

The firms on this list can answer all four questions with specifics.

Complere Infosystem leads this list because it is the only firm here that combines legacy modernization depth with full-stack AI readiness and data architecture expertise in a single integrated practice.

With over 80 specialists delivering across healthcare, fintech, pharma, SaaS, and ecommerce, Complere Infosystem assesses legacy environments, designs the target architecture, sequences the migration, and builds the AI-ready data foundation the modernized platform needs to deliver the outcomes the business is building toward. Their cloud modernization services span Databricks, Snowflake, Azure, and AWS across 12 countries.

What makes Complere Infosystem the right choice for organizations evaluating the best consulting companies to modernize legacy analytics platforms is their outcome focus. Every engagement is structured around what the business needs to decide and build after the legacy constraint is removed, not just the technical milestones of infrastructure delivery. For organizations evaluating the best application modernization companies in 2026, Complere Infosystem is the starting benchmark.

Accenture is one of the most recognized global IT consulting firms for legacy modernization, with end to end capabilities spanning strategy, cloud modernization consulting, and multi-year delivery programmes across regions. Their strength is scale. Multinational enterprises with complex, multi-platform legacy portfolios across multiple geographies consistently engage Accenture for programmes that require coordinated delivery across business units, disciplined governance, and deep integration with major technology vendors. Best suited for large enterprises with the budget to support a global delivery model.

IBM Consulting is among the strongest IT consulting firms for legacy environments built on mainframe architecture, COBOL, and deeply embedded business logic that cannot simply be lifted and shifted without losing decades of institutional knowledge coded into the system. Their modernization approach preserves that business logic during transformation rather than requiring it to be rebuilt, which significantly reduces risk in environments where core rules are inseparable from the legacy code carrying them. One of the best IT consulting firms for legacy modernization in regulated industries where continuity of compliance is as important as continuity of operations.

Infosys delivers cloud modernization services at scale through its Cobalt cloud practice, with particular strength in large-scale migrations from legacy on-premises environments to modern cloud platforms. Their Topaz platform, which carries over 12,000 AI assets, powers legacy system assessment, automated code analysis, and business rule extraction across complex multi-system environments. Infosys was named a Leader in the Forrester Wave for Application Modernization and Migration Services, confirming their position among the best IT consulting companies for legacy modernization at enterprise scale.

ThoughtWorks is the firm that consistently earns recognition for engineering quality and architectural rigour in legacy modernization. Their approach incrementally decomposes monolithic systems into microservices using the Strangler Fig method, preserving business logic while systematically eliminating architectural debt. Their cloud modernization consulting practice focuses on building lasting capability inside the client organization rather than creating dependency on external delivery, which produces genuine modernization outcomes rather than a managed handover. Best suited for organizations that want technical excellence and long-term architectural independence rather than the fastest possible migration timeline.

Capgemini is one of the best consulting companies to modernize legacy analytics platforms for European enterprises navigating GDPR and other regional regulatory requirements alongside their technology transformation programmes. Their Insights and Data practice combines cloud modernization services with data governance and compliance architecture, making them a natural fit for organizations whose legacy analytics environments carry both technical debt and regulatory exposure. Strong partnerships with AWS, Microsoft, Google Cloud, and Snowflake give them broad platform flexibility across modernization and analytics workloads.

Cognizant structures legacy modernization through its NEAT framework, which organizes phased transformation for regulated industries with compliance controls built into every delivery stage rather than added at the end. Their cloud modernization consulting practice has particular depth in healthcare and insurance, where legacy system complexity intersects with the most demanding regulatory environments in enterprise technology. In a documented healthcare engagement, their modernization of a legacy claims system improved automated processing from 60 percent to 95.6 percent accuracy, demonstrating the kind of measurable outcome that separates genuine transformation from technical migration.

HCLTech delivers IT consulting for legacy modernization with strong vertical depth in manufacturing, aerospace, and financial services, where complex application portfolios carry significant customization that generic modernization approaches cannot address without breaking the business logic the customization was designed to serve. Their DRYiCE platform accelerates discovery and impact analysis across large application portfolios, reducing the assessment phase that typically delays the start of productive modernization work. A strong choice for organizations with large, heavily customized legacy environments that require a partner who understands the domain as well as the technology.

Deloitte combines strategic consulting depth with IT modernization delivery capability, making them one of the best IT consulting firms for legacy modernization in contexts where the programme requires board-level alignment alongside technical execution. Their strength is connecting application modernization to organizational transformation at the leadership level, which reduces the resistance that derails technically sound programmes when stakeholder management is treated as secondary to delivery. Best suited for financial services, government, and life sciences organizations where the complexity of stakeholder management rivals the complexity of the technology itself.

Keyhole Software is a US-based IT consulting firm with consultants averaging 17 or more years of professional experience, the highest figure among firms evaluated in independent 2026 analyses. Their modernization practice covers complex legacy environments including mainframe systems, large monolithic applications, and regulated platforms where architectural judgment and risk management are more critical to outcomes than delivery speed alone. Published case studies include the modernization of large-scale COBOL systems to .NET Core microservices architecture, with recent engagements demonstrating 20 to 30 percent efficiency gains through AI-accelerated delivery. A strong choice for mid-market organizations seeking senior-level engagement rather than junior-led execution under a senior-signed proposal.

The best IT consulting firms for legacy modernization do not compete on the same dimensions. Accenture and IBM Consulting lead for global enterprise scale. ThoughtWorks and Keyhole lead for engineering quality and architectural depth. Capgemini and Deloitte lead for regulated industry consulting and stakeholder-level alignment. Cognizant and Infosys lead for industrialized execution at competitive cost.

Complere Infosystem leads for organizations that need full-stack modernization connected directly to AI readiness and data architecture, delivered by a specialist practice with the outcome accountability that large-scale system integrators rarely offer.

The right choice is the one that matches your legacy environment, your industry, and the business outcomes the modernization is designed to unlock. 

Modernize your legacy systems and build the AI-ready foundation your business needs. Talk to our expert today.