[2] He was a musical prodigy and was playing in the orchestra of Niblo's Garden in February 1864 before he was nine years old.
He left New York City for Boston where he played at the Bijou Opera House.
He turned to conducting and led the Germania Orchestra for several years and then led several annual tours as head of the Boston Festival Orchestra, leading concerts with such notable soloists as Emma Calvé, Nellie Melba, and Eugène Ysaÿe.
Mollenhauer was conductor of the Apollo Club of Boston, an acclaimed men's chorus, from November 1901 through April 1927.
[3] He became conductor of Boston's Handel and Haydn Society in 1899[2] and held that post until his death.