Following short spells in Geneva and on the NATO desk in Whitehall, Rycroft spent four years at the British embassy in Paris.

[citation needed] In 1995–96, Rycroft was Head of Section in the Eastern Adriatic Unit at the FCO: a demanding role, given the aftermath of the Yugoslav Wars.

In 2002, Rycroft was appointed Private Secretary to Prime Minister Tony Blair, to advise him on matters related to foreign policy, the European Union, Northern Ireland and defence.

[5][6] Rycroft's name became familiar to the general public as the author of a secret memo to the British Ambassador to the United States, David Manning, summarizing a 23 July 2002 meeting with Blair and other government officials "to discuss Iraq".

[7] The memo includes discussion of a "shift of attitude" in the Bush administration which made it appear that at this point, while the public was still being told that Iraq could avoid an invasion by agreeing to abide by UN resolutions, Military action was now seen as inevitable.Furthermore, the memo went on to state, Bush wanted to remove Saddam, through military action, justified by the conjunction of terrorism and WMD.

Saddam was not threatening his neighbours, and his WMD capability was less than that of Libya, North Korea or Iran.Rycroft was appointed Permanent Secretary at the Department for International Development with effect from January 2018.

He was appointed Permanent Under-Secretary of State of the Home Office in March 2020 following the resignation of Sir Philip Rutnam.