The bank, 'The Kumbakonam Bank Limited' as it was then called was incorporated as a limited company on 31st October, 1904. The first Memorandum of Association was signed by twenty devoted and prominent citizens of Kumbakonam including Sarvashri R. Santhanam Iyer, S.Krishna Iyer, V.Krishnaswami Iyengar and T.S.Raghavachariar. T.S.Raghavachariar was the First Agent of the Bank. In 1908, he was succeeded by Shri R. Santhanam Iyer who became the Secretary of the bank under the amended Articles of Association which created the office of a Secretary to be in charge of the Bank's Management in the place of the Agent, which post he held till his death in 1926. He was succeeded by Shri. S. Mahalinga Iyer as Secretary who subsequently became the First full-time Managing Director of the bank in tune with the amendment of Articles in 1929. He held the position of Secretary from 1926 to 1929 and that of Managing Director from 1929 to 1963. The bank in the beginning preferred the role of a regional bank and slowly but steadily built for itself a place in the Delta District Thanjavur. The first Branch of the Bank was opened at Mannargudi on 24th January 1930. Thereafter, branches were opened at Nagapattinam, Sannanallur, Ayyampet, Tirukattupalli, Tiruvarur, Manapparai, Mayuram and Porayar within a span of twenty five years. The Bank was included in the Second Schedule of Reserve Bank of India Act, 1934, on 22nd March 1945. The Bank celebrated its Golden Jubilee on 14th November, 1954 at Kumbakonam under the Presidentship of Shri.C.R.Srinivasan, Editor, 'Swadesmitran' & Director, Reserve Bank of India. In 1957, the bank took over the assets and liabilities of the Common Wealth Bank Limited and in the process annexed to it the five Branches of Common Wealth Bank Limited at Aduthurai, Kodavasal, Valangaiman, Jayankondacholopuram and Ariyalur. In 1963, Shri. R. A.Venkataramani Iyer took charge as the Chairman of the Bank which position he held up to 1969. In April, 1965, two other local banks viz., 'The City Forward Bank Limited' and 'The Union Bank Limited' were amalgamated with the Bank under a scheme of amalgamation with the resultant addition of six more branches viz., Kumbakonam-Town, Nannilam, Koradacherry, Tiruvidaimarudur, Tirupanandal and Kuttalam. Consequently, the Bank's name was changed to 'The Kumbakonam City Union Bank Limited'. It was later renamed as City Union Bank.
City Union Bank has now a strong network of 525 computerized branches and 1292 ATMs which is spread throughout India. In December 2006 Larsen & Turbo bought 10% of the bank. The bank celebrated its 110th years of service on 31st October 2014.