Lennig studied at Bruchsal under the private tutorship of the ex-Jesuit Laurentius Doller, and afterwards at the bishop's gymnasium at Mainz, his birthplace.
Being too young for ordination, he went to Paris to study Semitic languages under Sylvestre de Sacy, then to Rome for a higher course in theology.
In June 1832, he accepted the pastorate of Gaulsheim, now part of Bingen am Rhein, declining to take the chair of theology and exegesis at Mainz.
In 1852 he was made vicar general by Bishop Wilhelm Emmanuel von Ketteler, and in 1856 dean of the chapter.
Lennig published in 1849 his "Panegyric on Bishop Kaiser", and in 1862 his "Funeral Oration on the Archduchess Mathilde of Hesse".