The A555 was allocated to the Barnet Bypass on 1st April 1935, as part of the 1935 Road numbering revision. The bypass had opened in 1926 with the numbers A5093 south of the slightly earlier Watford Bypass (which opened as A5088 and became the A500 in 1935) and A5092 to the north. However, following the widespread introduction of Pre Worboys direction signs, it was decided to give the a more distinctive number to the road to aid navigation.
In 1937, when Trunk Roads were introduced, it became part of the London - Edinburgh - Thurso trunk route, while the A1 between Highgate and Hatfield remained a locally funded road. Around 1954, a renumbering scheme around London took place which reallocated principal route numbers along trunk roads where possible, and the road became part of the A1 proper. North of South Mimms the A1 has been upgraded to motorway (as the A1(M)); at times this was an online upgraded and at times the motorway was built slightly to one side: the original line of the A555 around Hatfield is now the A1001.