He played college basketball for the Adamson Soaring Falcons of the University Athletic Association of the Philippines (UAAP) from Seasons 81 to 86.

[3] These would be tournaments in which he would play basketball against other amateurs or even ex-pros for an agreed-upon fee, depending on the player's experience and talent level.

[2] After playing for Asian College - Dumaguete, he committed to Adamson and promised his parents that he would obtain a degree (sports management) in Manila.

[10] He rose to prominence as a player when he converted a clutch triple to force overtime against the UP Fighting Maroons which Adamson survived for a few minutes in the semifinals.

[14] He had a costly offensive foul against UP when he stuck out his leg on a three-ball attempt in their final possession, allowing UP to seal another win over them.

[17] He made 11 out of his 14 points in the first quarter, including a three-point shot that gave Adamson a commanding 16–2 cushion halfway through the period that they never looked back from.

[22] Against NU, he tallied 17 points on an efficient 6-of-8 shooting clip with five rebounds, four assists, a career-high three blocks, and two steals.

[27] Despite getting offers from Korean Basketball League (KBL) teams, Lastimosa opted to stay with Adamson for another year.

[29] He dropped a career-high of 29 points when the Soaring Falcons edged the De La Salle Green Archers in an overtime win in the first round of Season 85 on October 22, 2022.

[37] He then made two clutch free throws with 12 seconds left in the game as Adamson held on for the win, and got their Final Four spot and a matchup with Ateneo.

[42] Lastimosa suffered a partial ACL tear in his left knee in the weeks leading up to the start of UAAP Season 86.

[50] After a solid debut game in the Hotshots' convincing win over the Blackwater Bossing in the 2024–25 PBA Commissioner's Cup on November 28, Lastimosa performed his best night of 27 points, eight rebounds, and eight assists in a losing cause against the NorthPort Batang Pier on December 4.

[51][52] Having played abroad in several events, the Adamson star set foot in the 12-man Gilas Pilipinas for the 32nd Southeast Asian Games.

[53][54] On May 16, 2023, the national team went up once again against host Cambodia for the gold medal match after ousting defending champion Indonesia, 84-76, behind the heroic efforts of Justin Brownlee, Brandon Ganuelas-Rosser, Arvin Tolentino, and Lastimosa.