Rusnák played youth football in Slovakia for MFK Košice from a very young age, before signing for Manchester City in July 2010,[3] where he joined the club's reserve side.

[4] Rusnák made his first senior appearance as a 63rd-minute substitute for Jonson Clarke-Harris in a league game against Tranmere Rovers on 31 August 2013, in a match that they lost 0–1 at Boundary Park.

[6] On 17 September, Rusnák's loan was cut short and he returned to City after Rusnak reported refused to play in a reserve team match against Port Vale.

[12] Earlier in the season, in another local derby at SC Heerenveen, he gave the ball away from kick-off, resulting in a penalty being awarded after nine seconds, but Cambuur fought back for a 2–2 draw.

[13] On 3 May 2015, he scored both goals in the KNVB Cup final as they defeated reigning champions PEC Zwolle to win their first major trophy and qualify for the UEFA Europa League.

[20] On 10 November 2023, playing his more natural attacking midfielder role in the MLS Cup Playoffs, Rusnak scored the only goal in 1-0 home win in game 3 of Round One against FC Dallas.

[21] Being able to represent the Czech Republic due to being born there, nonetheless Rusnák made his debut for Slovakia on 15 November 2016, in the last friendly fixture for the year, against Austria, in a goal-less tie in Vienna, coming on as a substitute in the 46th minute for Jakub Holúbek.