Lechler was born on 26 July 1824 the son of a Pastor and Pietist Gottlob Lechler. He initially joined the merchant industry as an apprentice. During this time, an illness in which he contracted resulted in his Christian faith being strengthened and in 1844 committed himself to becoming a missionary by entering a Mission school run by the Basel Mission. It was during this time that he became acquainted with Theodore Hamberg During this time, the missionary Karl Gützlaff began touring churches in Western Europe and calling for missionary work to begin on the inland areas of China where no Westerner had ventured before. The Rhenish Missionary Society and the Basel Mission heeded the call and arranged for Theodor and Lechler to set sail for China, arriving on March 19, 1847.
In 1838, the Basel Missionary residing in India, C. Krückeberg travelled to Macau to recuperate from an illness and during this time he met Karl Gutzlaff and began to take an interest in the mission work taking place in China. He recommendeded that the Basel Mission society also begin sending its workers to China. On the May 13, 1846, the Basel Mission commissioned pastors Theodore Hamberg and Rudolph Lechler to China. On the same year, the Barmen Mission resolved to send Heinrich Köster and Ferdinand Genähr to assist Gutzlaff. The four of them reached Hong Kong on March 19. The Berlin Missionary Society also sent pastors Newman and Schubert as missionaries to China. In the early year, the three mission societies had their own separate mission goals - Gutzlaff arranged for the Basel Church to be responsible for evangelizing the eastern Guangdong region, Lechler was to administer to the Teochew people in the Chaozhou and Shantou areas, whereas Hamberg was minister to the Hakka population. The Barmen Mission workers were to be assigned the southeastern and western parts of the province where the language of the local population is predominantly Cantonese. The Berlin Mission Society was allotted the Guangzhou area north of the Beijiang River to preach to both the local Hakka and Cantonese residents. This strategy continued until after the Boxer Incident when all three mission socieites began to cooperate and jointly develop common goals amongst themselves.