When Kid Trunks and Kid Mai (who had been turned back into a little girl by the Dragon Balls after wishing to be younger) first meet Trunk lied about dating Mai while bragging to Goten. When Goten tried to call him out on it he convinces Mai to go along with the ruse. This seemed weird to her at first because she still had her adult memories despite reverting to the mentality of a child. Later, the two are shown bonding in a tree.
He develops a crush on her later on and is jealous of her crushing on his future counterpart so he tries to impress her. He attempts to act like Future Trunks. This doesn't appear to work well.
In Resurrection F, Mai states that Trunks is her boyfriend.
In the alternate timeline Future Trunks and Future Mai are in a relationship. Even against the power of Goku Black, Mai manages to find ways to support Future Trunks. She helps him escape death once when he is beaten with smoke screen missiles. Future Trunks morns when he thinks she has been killed, and is greatly relieved when he finds her alive. After the defeat of Goku Black and Zamasu, Bulma states how her future self might become a grandmother soon, much to Future Trunks and Future Mai's embarrassment.
The ship has some followers due to their time spent together in Dragon Ball Super. Most fanfics take place in the alternate timeline as they fight Goku Black. It rivals TruPan.