Insights
The deal creates the value on paper. The first hundred days decide whether you keep it. A practical guide from someone who has bought, built, sold and separated businesses, not just advised on them.
By Christopher Fagan, PEF Consulting Partners. 3 September 2026.
Every acquisition case assumes things that have not happened yet: synergies captured, customers retained, key people staying, systems separated or combined on schedule. None of those happen in the data room. They happen, or fail to happen, in the first hundred days after completion, when attention is highest, goodwill is freshest, and the organisation is most willing to accept change. Miss the window and every later change costs more, because by then the acquired business has settled back into its old shape with a new logo on it.
Having built and sold businesses of my own, led merger and acquisition activity at Smart Choice Metering, and delivered the operational separation of National Grid and NESO, my view is that most integrations do not fail from bad strategy. They fail from unowned decisions and untested assumptions, both of which are cheap to fix in the first hundred days and expensive to fix afterwards.
Day one should be planned like a go live, because that is what it is. People need to be paid, customers served, orders fulfilled and regulators satisfied from the first morning, across two organisations that have never operated as one. The day one plan is a short list of things that must work, each with a named owner and a tested fallback. Everything else can wait. A business that gets day one visibly right buys itself months of credibility with staff and customers; one that fumbles payroll or invoicing in week one spends the rest of the integration earning back trust.
Who is in charge, of what, by name. Ambiguity at the top of an acquired business is a resignation letter factory. Decide the leadership and reporting lines before completion and announce them at once, including the uncomfortable ones.
What you will not integrate. Full integration is not the goal; value is. Some functions, systems and brands should be left alone, some combined, some retired. Making the keep, combine or kill decision explicitly, per function and per system, prevents two of everything running indefinitely, which is where synergy cases quietly die.
Where the money story lives. From day one you need one version of financial truth across both businesses, even if it is assembled manually at first. Every week without it, the synergy tracking, the covenant reporting and the board narrative drift apart.
Which people you cannot afford to lose. The org chart tells you who is senior. It does not tell you who holds the customer relationships, the system knowledge and the informal authority. Find those people in the first two weeks and give them a reason to stay, because your competitors are calling them.
What the customers will be told. Customers do not care about your integration. They care whether service holds and prices change. A clear, honest customer story, delivered by the people they already know, protects the revenue the whole deal is priced on.
The technology workstream is where hundred day plans most often meet reality. Separations have contractual deadlines and transitional service agreements that get expensive when they overrun. Integrations inherit two estates, two data models and two sets of undocumented exceptions. The discipline is the same one that governs any migration: understand the data before you move it, decide the target honestly, move in controlled steps, and reconcile everything. On the National Grid and NESO separation, the operational work stood or fell on exactly this discipline, applied under a deadline that could not move.
A hundred day plan is a delivery programme wearing a corporate finance name. It needs an owner with real authority, a cadence that surfaces problems in days rather than months, and a scorecard the board actually reads. The acquirers who do this well treat integration leadership as a senior, accountable role, not a task force bolted onto day jobs. The ones who do it badly discover at month six that everyone assumed someone else owned the synergy case.
Day one readiness, the decisions that cannot wait on leadership, structure and customers, a plan for systems and data, communication to staff and customers, early value from the deal case, and governance that runs the integration as a programme with named owners.
PEF Consulting Partners leads post acquisition delivery from the client side: day one readiness, integration and separation programmes, and the first hundred days that decide the deal. A short, confidential conversation before completion is worth more than a rescue afterwards.