Capitalize titles of persons when used as part of a proper name. Generally, when the title is used alone, it is not capitalized.
Prof. Margaret Barnes; Dr. Sinyee Sein; John Scott Williams Jr.; Anne Tilton, LLD
Usage varies when the title of an important public figure is used alone.
Exercises:
Capitalization 1
Capitalization 2
Related topic:
Abbreviations of titles with proper names