According to Wikipedia, at least (I know, not the best source, but first I found in a quick google search):
A wife who uses the title Mrs. would also use her husband's full name, including the suffix. In less formal situations, the suffix may be omitted. Hence: Mrs. Lon Chaney Jr. on a wedding invitation, but Mrs. L. Chaney or simply Shannon Chaney for a friendly note. Widows are entitled to retain their late husband's full names and suffixes, but divorcees may not continue to style themselves with a former husband's full name and suffix, even if they retain the surname.
Further, it is more correct to name the wife's name first, then the husbands. (see Crane's Blue Book of Social Stationery)
As such: "Mary and John Smith Jr." (or alternatively: "Mary and John Smith, Jr.")
Note: With regards to whether to use a comma or not when using Jr., Sr., etc, see this Chicago Manual of Style page.