Haverfordwest are back in the Welsh Premier League after a four-year absence following a 5-0 Welsh League win at Aberdare.
Cardiff club Caerau have won the Welsh League, but do not have the criteria to be promoted.
Luke Borelli's early brace was followed by goals from Dale Griffiths, Steffan Williams and Sam Roden.
Those goals ensured Haverfordwest finished second by scoring 69 goals to third-placed Cardiff Met's 57.
Haverfordwest and the students were tied on an equal goal difference of 37.
Llandudno had already secured their promotion to Welsh football's top domestic tier from the Cymru Alliance.