On 28 April 2011, Petry was brought on as a substitute in the Danish Cup semi-final win over Randers, his first appearance for the first team.[2] He signed a new contract on 2 April 2012, extending his contract with Nordsjælland to June 2014.[3] The following years, however, were plagued by injuries which meant that Petry never established himself in the starting lineup. Between 2014 and 2017, he only made 33 league appearances due to injuries, mainly a meniscus tear which kept him sidelined for an extended period of time.[4]
In May 2018, Nordsjælland announced on their website that Petry would leave the club when his contract expired on 30 June.[5] On 18 August 2018, Petry signed a six-month contract with Lyngby Boldklub.[6] He made 14 appearances in which he scored one goal during his tenure at Lyngby.[7]
On 7 January 2019, Petry moved to defending Icelandic champions Valur alongside fellow Dane Emil Lyng.[8]
Petry returned to Denmark at the end of the season and signed with Danish 1st Division club HB Køge on 30 December 2020.[9] He made his debut for the club on 13 February 2021 in a 1–1 draw against Hobro IK. In the game, he scored the equaliser for Køge in the 83rd minute, becoming the 12th player in club history to score in his debut.[10][11]
In early May 2022, Petry terminated his contract with Køge and returned to Iceland, where he signed for FH Hafnarfjördur.[12] With the dismissal of Ólafur Jóhannesson and his subsequent reinstatement as head coach of Valur, Petry found himself again playing for Valur. Due to persistent knee problems, 30-year-old Petry confirmed on 8 November 2022 that he would retire with immediate effect.[13]
^Jensen, Kenneth (26 October 2017). "Klarmeldt Lasse Petry har kontraktsnak kørende med FCN" (in Danish). Tipsbladet. Retrieved 13 October 2020. Siden sommeren 2014 har Lasse Petry bare spillet 33 Superliga-kampe for FC Nordsjælland, og det er hovedsageligt på grund af skader. I 14/15 blev det til 13 kampe, 15/16 15 kampe og i 16/17 fire kampe. I denne sæson er det blevet til en enkelt Superliga-kamp for Petry, der er vendt tilbage efter en menisken-operation.