RB Leipzig

After beginning his career with spells in the youth systems at Bargfelder SV, SSC Hagen Ahrensburg and Hamburger SV, Janelt joined the academy at RB Leipzig in 2014,^[3] for a fee reported to be €150,000.^[4] He signed a five-year professional contract in July 2016 and progressed to the reserve team,^[5] for which he made six Regionalliga Nordost appearances during the 2016–17 season.^[2] Prior to Janelt's departure from the Red Bull Arena at the end of the 2017–18 season,^[3] disciplinary issues led to him spending 18 months away on loan.^[4][6][7] He failed to win a call into a first team matchday squad during his time with RB Leipzig.^[2]

VfL Bochum

On 9 January 2017, Janelt joined 2. Bundesliga club VfL Bochum on loan until 30 June 2018,^[6] with an option to buy.^[8] He made 20 appearances during an injury-affected spell and signed a three-year contract with the club on 30 May 2018,^[2][8] for an undisclosed fee.^[3] Janelt looked set to make a breakthrough late in the 2018–19 season, before a torn adductor ruled him out of the final eight matches of the campaign.^[9] He broke into the first team in 2019–20, making 24 appearances during a season which was ended prematurely by the COVID-19 pandemic.^[2]

Then in the final season of his contract,^[10] Janelt departed VfL Bochum in October 2020 and ended his 3+1⁄2 years at the Ruhrstadion with 54 appearances.^[2][11] In December 2021, Janelt stated that he "struggled to find consistency" during his spell with the club,^[12] exacerbated by playing under six different managers.^[13] He was frequently deployed as a utility player, filling in as a left winger, left back, central defender and on one occasion,^[13] as a stand-in goalkeeper late in a match.^[14]

Brentford

On 3 October 2020, Janelt moved to England to sign a four-year contract with Championship club Brentford for an undisclosed fee of "around" £500,000.^[11][13] Despite being earmarked by co-director of football Rasmus Ankersen as an overseas player who would "need time to adapt to English football",^[11] an injury suffered by first-choice defensive midfielder Christian Nørgaard in the days following the transfer allowed Janelt to break into the matchday squad.^[14] By late October, he had assumed Nørgaard's starting role.^[2][15] Janelt retained his place throughout the campaign and finished Brentford's 2021 EFL Championship play-off final-winning 2020–21 season with 47 appearances and four goals.^[16][17]

Janelt began the 2021–22 season as an ever-present starter in Premier League matches and he scored his first goal of the season in a 3–3 draw with Liverpool on 25 September 2021.^[18] Aside from missing one month due to a thigh injury suffered in October 2021,^[18][19] Janelt continued in his virtual ever-present role and signed a new four-year contract on 1 April 2022.^[20] The following day, he scored his third and fourth goals of the season in a 4–1 win over Chelsea and finished the campaign with 35 appearances.^[18]

Janelt began the 2022–23 season primarily in a substitute role and he captained the club for the first time during a 2–0 EFL Cup second round win over Colchester United on 23 August 2022.^[2][21] Despite airing some concerns about his playing time,^[22] Janelt finished the season with 37 appearances (predominantly as a starter) and three goals.^[2]

Janelt began the 2023–24 season as an ever-present in midfield all competitions.^[23] Following the loss of first-choice wing backs Rico Henry and Aaron Hickey due to injury, he deputised at left wing back on an ad-hoc basis.^[24][25]