Tanush Karunakar Kotian was born on October 16, 1998, in Chembur, Mumbai, Maharashtra. Raised in a family with deep roots in Pangala village, Udupi district, Karnataka, Kotian developed his game through the Mumbai age-group circuit while attending Ramniranjan Jhunjhunwala College. Operating as a right-handed lower-order batter and right-arm off-break bowler, he earned his early international recognition when he was selected for the India Under-19 team in 2017.

His senior domestic career began in December 2018, making his First-Class debut for Mumbai against Saurashtra in the 2018-19 Ranji Trophy. He went on to make his List A debut on March 9, 2021, against Saurashtra in the Vijay Hazare Trophy, followed by his T20 debut in 2021, against Karnataka in the Syed Mushtaq Ali Trophy. Over his initial seasons, Kotian established a reputation as a composed off-spinner capable of providing crucial control and delivering resilient lower-order runs under pressure.

Kotian experienced a historic breakthrough during the 2023–24 domestic season. He played a central role in Mumbai’s 42nd Ranji Trophy victory, scoring 502 runs at an average of 41.83 and taking 29 wickets at 16.96 to earn the Player of the Tournament award.

 During the quarter-final against Baroda, Kotian scored his maiden First-Class century (120* off 129 balls) and shared a historic 232-run 10th-wicket partnership with Tushar Deshpande, making them the first No. 10 and No. 11 batters to score centuries in the same Ranji Trophy innings. Shortly after, he joined the Rajasthan Royals as a replacement for Adam Zampa and made his Indian Premier League debut on April 13, 2024, against Punjab Kings.

In October 2024, he struck an unbeaten 114 in the second innings of the Irani Cup against Rest of India, helping Mumbai end a 27-year title drought and earning the Player of the Match award. In December 2024, the Men's Selection Committee added Kotian to the senior Indian Test squad for the final two Test matches of the Border-Gavaskar Trophy in Australia following Ravichandran Ashwin's international retirement. Heading into the 2025-26 cycle, Kotian continued his strong form for India A against South Africa A and featured prominently for the North Mumbai Panthers in the T20 Mumbai League.

During the 2026–27 Duleep Trophy quarter-final against North Zone in August 2026, Kotian delivered an outstanding bowling effort, returning figures of 4/72 in the first innings and 4/46 in the second innings. In September 2026, he was named in the India A multi-day squad for the home series against Australia A in Puducherry. In the first unofficial Test, he played a crucial counter-attacking knock, scoring a half-century (68) in the first innings to anchor India A's batting effort.