Twenty years after the Civil War ended, St. Mark's United Church of Christ was born. On January 2, 1885, members of First (Tabor) Reformed Church at 10th and Walnut streets in Lebanon decided to establish a Sunday School in the northern part of the city. Two months later, on March 1, 1885, St. Mark's Sunday School was organized by the Rev. F.W. Kremer, who led a group of 65 adult charter members to a room of the U.B. Aid Building at North Ninth and Scull streets. Later that year, on December 13, land was purchased at North Eighth and Mifflin streets, where a chapel was built and dedicated during the Advent season. As a token of its sibling love, Tabor presented the new Sunday School with one of its own small bells.
On May 12, 1887, St. Mark's Reformed Church was formed, and the Rev. Oscar P. Steckel received a call to be the congregation's first pastor. During Steckel's five years as pastor, the first pipe organ, built by Miller Organ Company of Lebanon, was installed and a parsonage was erected east of the chapel.
The Rev. I. Calvin Fisher followed Steckel as pastor and served from 1892 until 1932. On June 16, 1901, the church's present sanctuary was dedicated. In 1908, St. Mark's bought the M.E. Church chapel north of Lebanon and established its own mission Sunday School, Calvary Reformed Sunday School of Reinoeldville. The mission remained part of St. Mark's until June 1954 when Calvary was handed full title to the property and became part of the Calvary-Hill charge.
The three buildings of St. Mark's - church, chapel and parsonage - served the congregation well through the pastorates of the Rev. Henry J. Herber (1933-1952) and the Rev. Ralph D. Althouse (1952-1963).
In 1968, under the leadership of the Rev. George Shultz Jr., who had received his call in 1964, the chapel and parsonage were replaced with a Christian Education building. A second building program, in 1973, included a complete renovation of the sanctuary, the addition of a narthex and the installation of a new pipe organ. Church offices, community rooms, an elevator and a new entrance were added in 1991 to complete the extensive expansion and renovation.
The Rev. Richard F. Luciotti was elected as St. Mark's sixth pastor on May 15, 1994, 16 months after Shultz retired.
St. Mark's has had several associate and assistant ministers during the past five decades. The Rev. Howard Schley Fox served St. Mark's as parish minister from 1966 until 1973. The Rev. David W. Minnick served as associate minister from 1978 until 1983. The Rev. Rebecca J. Seldomridge followed Minnick as associate minister from 1984 until 1988. The Rev. Cynthia Garis served as associate pastor from October 1998 until her resignation in 2004.
In July 2016, St. Mark's, through a cooperative ministry with St. John's UCC, Lebanon, called its seventh pastor, the Rev. David A. Jones, who serves both congregations today.
St. Mark's welcomed a future United States president on April 20, 2008 when Sen. Barack Obama of Illinois attended a worship service. The visit was unannounced to the public and took place two days before the Pennsylvania Democratic primary. Obama was elected president in November 2008. He was the first presidential candidate to visit Lebanon since Sen. John F. Kennedy in 1960.
In 2009, St. Mark's celebrated the 50th anniversary of its Pine Acres facility along Route 322 in the hills of South Lebanon Township. Pine Acres is used for the annual church picnic in August and for the annual Kid's Kamp program every July. Church members and outside groups also use Pine Acres for youth events and social functions. A dedicated group of volunteers maintains Pine Acres throughout the year. In January 2010, the congregation voted to purchase two homes just north of the church on North Eighth Street. The homes were demolished to create additional parking for church activities.