Answer

No. Vardhaman Mahavir was not a king.

He was born as a prince in a royal Kshatriya family. His father was King Siddhartha and his mother was Queen Trishala. Later, Vardhaman renounced worldly life and became a monk, and ultimately attained Keval Jnan and became the 24th Tirthankar, Bhagwan Mahavir.

Takeaway

Vardhaman Mahavir was a prince by birth, but he gave up royal comforts to follow the path of renunciation, non-violence, and liberation.