The 24-year-old Swiss striker has been granted international clearance to complete his move to The Jakemans Stadium after spending several months in the League of Ireland.
Igiehon had top-scored for Scarborough Athletic in Evo-Stik League North last season before securing a move into the Irish professional game with Sligo.
Formerly with Swiss sides FC Solothurn and FC Langenthal, he joined Alfreton Town in September 2014 and then had spells with Bradford Park Avenue and Sheffield FC before joining Scarborough in the summer of 2016.
He has returned to England as a free agent and is eligible to make his United debut against Leamington on Saturday.