The deal is closed. The press release is out. And now the real work begins.

Research consistently shows that 70% to 90% of acquisitions fail to deliver their intended value - and it's rarely the deal itself that falls apart. It's what happens, or doesn't happen, in the days and weeks after close. The first 100 days set the trajectory for everything that follows: whether key talent stays, whether customers notice a difference, and whether the synergies in the deal model ever show up on a P&L.

This checklist breaks down what leadership teams need to have in place, organized the way Stonehill structures every post-merger integration engagement: across leadership, operations, systems, and culture.

Before Day 1: Set the Foundation

Integration planning shouldn't start at close - it should start before signing, if possible.

• Confirm the deal thesis is documented and shared. Everyone driving integration should understand exactly where the value is expected to come from: cost synergies, revenue synergies, capability gains, or some combination.

• Stand up an Integration Management Office (IMO). A single team needs to own coordination across workstreams, decision-making, and reporting. Without it, integration becomes a series of disconnected efforts.

• Identify Day 1 readiness requirements. Payroll, customer-facing systems, legal entity changes, and communications all need to function the moment the deal closes — not weeks later.

• Build the first communication plan. Employees, customers, and partners will fill any information vacuum with their own assumptions. Get ahead of it.

Days 1–30: Stabilize

The first month is about keeping the business running while the integration structure comes online.

• Confirm leadership roles and reporting lines. Ambiguity at the top creates ambiguity everywhere else.

• Launch the integration governance cadence. Regular check-ins across workstream leads, with clear escalation paths, keep small issues from becoming Day 60 fires.

• Audit critical systems and processes. Know what's duplicated, what's incompatible, and what's at risk of breaking before it actually breaks.

• Protect customer-facing continuity. Customers should feel no disruption in service, billing, or support during this window.

• Start tracking retention of key talent. The people who matter most are also the most likely to be evaluating their options right now.

Days 30–60: Align

With the immediate fires contained, focus shifts to designing how the combined organization will actually operate.

• Finalize the target operating model. One structure, one set of reporting lines, one governance model - across functions, not just at the top.

• Map operational and systems integration priorities. Decide what gets unified now, what gets phased, and what stays separate for good reason.

• Define and prioritize synergy targets. Cost and revenue synergies need owners, timelines, and tracking - not just a line item in the original deal model.

• Address cultural friction directly. Don't assume culture will "work itself out." Identify where the two organizations differ in how decisions get made, how work gets done, and how people communicate.

Days 60–100: Accelerate

By this point, the foundation should be in place to start driving toward the value the deal was built on.

• Begin executing synergy initiatives with accountability. Assign owners, set milestones, and track progress against the original deal thesis.

• Reassess organizational design based on early signals. What looked right on paper at Day 1 may need adjustment once the business is actually running combined.

• Formalize the cultural integration plan. Move from identifying friction points to actively building the combined culture leadership wants.

• Report progress against the deal thesis. Leadership and, where relevant, investors need visibility into whether the integration is on track to deliver what was modeled.

Why Most Integrations Lose Momentum Here

The first 100 days are where strategic intent either becomes operational reality - or quietly stalls. Teams get pulled back into day-to-day firefighting, the IMO loses authority, and synergy tracking falls off the agenda. The deal thesis that justified the acquisition gets shelved instead of executed.

That's the gap Stonehill closes. We provide post-merger integration consulting for private equity firms, corporate acquirers, and leadership teams who need to bring organizations together quickly without losing performance - aligning integration strategy and value capture, operating model design, systems integration, and cultural integration into one coordinated execution plan.

Use our free Post-Merger Integration Checklist to pressure-test your plan before Day 1, or talk to our team about how Stonehill can run point on integration so your leadership team can focus on running the business.