The Privy Council voted to increase its boundaries on March 8, 1850, making Thomas Square Hawaii's oldest city park.

On July 26, Admiral Richard Darton Thomas sailed into Honolulu harbor on his flagship HMS Dublin.

[5] By 1883, Cleghorn had approved Robert Stirling's park design, which laid out a series of circular and semicircular paths.

Since Honolulu's treasury was in "dire condition" at the time,[5] Cleghorn brought banyan trees from ʻĀinahau, his Waikiki estate, to plant and create more shade.

The improved park celebrated a successful grand opening on April 7, 1887 where the Royal Hawaiian Band performed to a huge crowd.

[3] In the early 1930s, the Parks Boards commissioned and adopted a renovation landscape plan by Catherine J. Richards and Robert O. Thompson.

The renovation included a mock orange hedge along the curbs, flower beds bisecting a central walkway, and a terraces and coral wall parallel to Beretania Street.

[9] It received $50,000 in 1966 for a renovation that included an expanded comfort station, a new coral walkway from Beretania Street, and tree-pruning to thin out the canopies to allow for more light and air.