Both the Main Street Branch Library and Triangle Park were listed on the National Register of Historic Places on April 13, 2013, by the United States Department of the Interior, Reference Number 13000157.

[4] James Edward McClellan, Denver Markwith, Jr., and Jack Hunt MacDonald, developed site-cast, concrete tilt-up construction, during the post-World War II building boom in Southern California.

[2] A handcrafted grandfather clock in the library's main room was donated by the senior class of Huntington Beach High School in 1914.

[6][7] A portion of Block 505, the future Triangle Park, was temporarily used for tents to house the constant flow of new residents to Huntington Beach upon the discovery of oil in 1920.

"[8] In 1927, the recreational improvements at Triangle Park were well underway, with the addition of a lighted baseball diamond, grandstand, and cement work on the horseshoe grounds.

In 1933, Triangle Park was considered as a possible site for a federal post office, but later rejected when a downtown Huntington Beach parcel on Main Street was identified.