Yes, according to Jain scriptures, Bhagwan Adinath (Rishabhdev) was a king before he became a Tirthankar.

• He was born as a prince to King Nabhi and Queen Marudevi in Ayodhya (Vinita).
• He later became a righteous Chakravarti-type king, teaching people farming, arts, crafts, and social order.
• After many years of ruling, he developed deep vairagya (detachment), gave his kingdom to his sons (like Bharat and Bahubali), and then renounced everything to become an ascetic.
• Through hard penance and meditation, he attained Keval Jnan (omniscience) and became the first Tirthankar of the present time-cycle.

So, in simple words: Yes – Adinath Bhagwan was first a great king, and then he left everything and became a Tirthankar.

You can read a detailed, simple story of his life here: