Lethbridge first signed for Pompey as a 10-year-old central midfielder but was released after just one year.
Returned as a striker in 2014, and was awarded a first-year apprenticeship in July 2017.
A powerful forward with an impressive goalscoring record at academy level, Lethbridge was one of the standout performers in the Under-18s' 2017 FA Youth Cup run to the third round.
Named in Pompey's squad to face Northampton in the Checkatrade Trophy second round in December 2017, as he remained an unused substitute.
Shone in the first round of the FA Youth Cup, netting twice as the young Blues ran out 5-2 winners over Bristol Rovers at Fratton Park, before making his full senior debut just three weeks later against Tottenham Hotspur U21s.
Played 65 minutes of the Blues' 3-2 Checkatrade Trophy group stage victory against Spurs' development squad on his professional bow.