Sim Mow-yu was born in the southern Malaysian city of Melaka, in a family dedicated to academia-his grandfather had been a scholar in the Qing Dynasty court in China, and his father was one of Sun Yat-sen's earlier followers, serving as head of the Melaka branch of Sun's Revolutionary Alliance, and later as a member of the overseas Chinese committee of the Kuomintang.
沈慕羽出生於馬來半島南端、古城麻六甲的書香世家，祖父是晚清秀才，父親沈鴻柏早年追隨孫中山先生，擔任革命黨同盟會麻六甲支部部長，之後中國國民黨又委任他為僑務委員。