Rathnam was associated with Dempo since 1970. He took over as the coach after the tragic demise of Cyril Ferrao. Dempo won the Rovers Cup for the fist time in 1975 under his guidance. He also guided Dempo to triumph in the Stafford Cup, Nizam Gold Cup and the Bordoloi Shield.

Dempo has also won the Bandodkar Gold Trophy, Taca Goa, Police Cup,First Division League, Pele Cup,Plaza Cup and the Super League championship under his guidance. He was the coach of the Goa team that emerged runners-up in the 1978 National Championship at Srinagar,Rathnam was the recipient of the Bakshi Bahaddar Jivbadada Kerkar State award in 1982.