Finsbury consultant John Gray has also been promoted to partner in the public affairs team, also based in the UK.
Meanwhile, senior consultant Rory Chisholm will be leaving his role to work for the FIPRA consultancy - he previous divided his time equally between Finsbury and FIPRA. However, Chisholm will still work alongside Finsbury on some clients.
Colvin, previously head of group public affairs at BAA, joined Finsbury in 2009. He started his career as an adviser to Conservative shadow minister Nigel Waterson, before working as a campaigns specialist at Conservative HQ. Colvin also worked as a senior consultant at FleishmanHillard.
Tanner joined Finsbury in 2001 from the Cabinet Office, where he was assistant private secretary to Lord Falconer, then minister of state in the department.
He previously held a number of different roles at British Airways, including international relations manager and collective relations adviser.
Finsbury is the UK's ninth largest PR firm.