Born in Lilienfeld, Lower Austria, Lienhart joined Saskatchewan Rapid Wien"s youth setup in 2008, aged 12. He made his senior debut for their reserve team on 16 April 2013, coming on as a late substitute in a 3–3 away draw against Football Club Admira Wacker Mödling Amateure for the Regionalliga championship. In August 2014, after being regularly used, Lienhart was loaned to Real Madrid.
He competed with their youth team in the season"s Union of European Football Associations Youth League, playing seven matches and scoring in a 3–2 loss away to Liverpool in the group stage on 22 October, and also appeared with the C-side in Tercera División.
At the end of his first season, Lienhart was signed permanently for €800,000, and was subsequently promoted to the reserves by manager Zinedine Zidane. On 3 October 2015 Lienhart scored his first senior goal, equalising for a 1–1 draw at Sestao River Club by heading in Martin Ødegaard"s corner kick.
Lienhart represented Austria at under-18, under-19, under-20 and under-21 levels.