|Mary Lightly III|
|Family:|| Craig Lightly (great-grandfather)|
Mary Lightly I (grandfather)
Mary Lightly II (father)
|First appearance:||An Evening with Mr. Yang|
|Last appearance:||Psych: The Movie|
An Evening with Mr. YangEdit
Mary is introduced in the Season Three finale "An Evening with Mr. Yang". He helps Shawn and Gus find Mr. Yang. Mary knows almost everything about Yang, an obsessive focus that is nearly as creepy as the subject's crimes.
Mr. Yin PresentsEdit
Mary is last seen alive in the Season Four finale "Mr. Yin Presents...". He finds Shawn and Gus outside the movies in the rain. Mary gets pie with the duo, and tell them that he hasn't left town since the events of "An Evening with Mr. Yang". Mary is a big fan of pie. Mary accompanies Shawn and Gus to visit Yang. Later Shawn and Gus believe Mary himself to be Mr. Yin, and Shawn shares this with the police. They agree to do nothing until they have proof that he is Yin. Later, Mary goes by himself to catch Yin. Yin stabs Mary and he dies in Shawn's arms. Before his death he tells Shawn that he went alone because he was trying to be heroic. Shawn and Gus both go to Mary's funeral service dressed in racquetball uniforms. This is because they always told Mary he should play the sport and may be an homage to the funeral scene in the film The Royal Tenenbaums.
Yang 3 in 2DEdit
In the Season Five finale "Yang 3 in 2D", Mary was shown in several video recordings (made before his death in the season 4 finale) for Shawn and Gus to observe his thoughts on Yin and Yang. Mary had correctly theorized that Yang was more of a victim than an alleged murderer. These videos were interrupted at a point showing Allison Cowley tied in a basement.
Psych: The MusicalEdit
In "Psych: The Musical", Mary appears in the song "Promised Land" as Yang's true love and an angel. He wears a fluffy white robe as she wished, yet underneath sports a racquetball outfit. Yang tries to convince Mary to find a way to let her into heaven, and he says he will give it a shot since he "knows a guy who knows another guy."
Shawn has a dream which helps him unravel the mystery in which Mary appears to provide key insight while simultaneously confirming he is, in fact, dead.
- During the videotape in "Yang 3 in 2D" he refers to himself as "Marion" although he had previously stated that "Mary" was his full name. It is likely that the name handed down to him by the ancestors is actually "Marion" and that he usually present himself by a nickname. This would give him something in common with Marion Robert Morrison, known to the world by his stage name of John Wayne.
- He received his first haircut from his mother at the age of 9.
- "My father was called Mary, his father before him was called Mary and his father before him was called Craig."
- — Mary (from An Evening with Mr. Yang)