In addition to newly appointed head coach John Gregory, Chennaiyin FC already had local favourite and former India international Syed Sabir Pasha as assistant coach and now Lillis and Clark join the revamped coaching staff.
"Both Mark and Niall are highly qualified and experienced in their respective fields of expertise. We are delighted to have them on board and believe that along with Sabir we have the best team to support John in our quest of winning back the ISL trophy," said Chennaiyin FC co-owner Mrs Vita Dani.
His coaching career started at Macclesfield Town before a spell at Halifax Town. Lillis worked with Gregory at Derby County but he has done most of his coaching at Huddersfield Town, where he has served as coach, caretaker manager and academy manager.
He also had a spell as assistant manager of Northern Ireland.
Clark has had spells with Manchester City, Crystal Palace, West Ham, Charlton Atheltic in England besides working with Gregory at English third tier outfit Crawley Town. In 2015, he worked with FC Pune City in the ISL and last season he played a key role in Kerala Blasters' road to the final.