Tan Sri Dato' Chang Ko Youn was called to the English Bar as a Barrister-At-Law of Lincoln Inn, London in 1982 and was subsequently called to the Malayan Bar in 1983. 

Tan Sri Dato' Chang Ko Youn began his career as a lawyer in Ipoh in 1983 until 1995 where he was attached to Chang Ko Youn & Co. In 1987, he was appointed as Councilor of the Kuala Kangsar District Council until 1995. Over the years, he has held numerous political positions in Parti Gerakan. In 1995, he was appointed as a Member of the Perak State Executive Council after winning the Jalong State Seat and held the post until 2008. In 2009, he was appointed as the Advisor of the Menteri Besar of Perak until 2013.

Since 2014, he has resumed his legal practice at Toh Theam Hock & Co in Ipoh, specialising in banking, commercial and conveyancing matters. He has been appointed as the Chairman of Yayasan Penjaja Dan Peniaga Kecil 1 Malaysia (YPPKM), a body handling micro-credit for petty traders sponsored by the Ministry of Finance. On 20 July 2022, he was appointed as the director for Perak Academy, a private funded organization that promotes interest in the state of Perak by encouraging discussion, scholarship and research. On 7 June 2023, he was appointed as the Trustee Holder for The Trustees of Malaysia Game Changer Foundation Registered.

Perak Transit Berhad

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