Mladenov started his career in his home town Kyustendil in the local team, Velbazhd.

On 30 June 2010, he made his unofficial debut for Levski in a match against FCM Târgu Mureş.

On 16 December 2010, Mladenov netted the only goal (right at the end of the first half) for Levski Sofia in the 1:0 home win against Sporting CP in the final UEFA Europa League match of the group phase.

On 2 June 2013 Mladenov played one match for Marek Dupnitsa in the amateur V Group against Slivnishki geroi and scored a goal.

[6] Mladenov earned his first cap for Bulgaria on 7 September 2019, coming on as a late second-half substitute for Galin Ivanov in the 0:4 away loss against England in a UEFA Euro 2020 qualifier.