Main characters
- Brigadier General Ezra Mannon
- Christine Mannon, his wife
- Lavinia Mannon – their daughter
- Orin Mannon – their son, First Lieutenant of Infantry
- Captain Adam Brant – of the clipper "Flying Trades"
- Captain Peter Niles – Orin's friend, from the U.S. Artillery
- Hazel Niles – his sister
- Seth Beckwith – the old family retainer and gardener
Chorus of townsfolk – (various chorus members appear in different scenes)
- Amos Ames – a middle-aged carpenter
- Louisa Ames – Amos' wife
- Minnie – Louisa's cousin
- The Chantyman
- Josiah Bordon – manager of the shipping company
- Emma – his wife
- Everett Hills, D.D. – of the First Congregational Church
- His wife
- Doctor Joseph Blake – a family physician
- Ira Mackel – an old farmer
- Joe Silva – a Portuguese fishing captain
- Abner Small – a little old clerk in a hardware store
The story is a retelling of the Oresteia by Aeschylus. The characters parallel characters from the ancient Greek plays. For example, Agamemnon from the Oresteia becomes General Ezra Mannon. Clytemnestra becomes Christine, Orestes becomes Orin, Electra becomes Lavinia, Aegisthus becomes Adam Brant, etc. As a Greek tragedy made modern, the play features murder, adultery, incestuous love, and revenge, as well as a group of townspeople who function as a kind of Greek chorus. Although fate alone guides characters' actions in Greek tragedies, O'Neill's characters also have motivations grounded in 1930s-era psychological theory. The play can easily be read from a Freudian perspective, paying attention to various characters' Oedipus complexes and Electra complexes.
Mourning Becomes Electra is divided into three plays with themes that correspond to the Oresteia trilogy. Much like the Aeschylus plays Agamemnon, The Libation Bearers, and The Eumenides, these three plays by O'Neill are correspondingly titled Homecoming, The Hunted, and The Haunted, respectively. These plays are normally not produced individually, however, but only as part of the larger trilogy. Each play contains four to five acts, with only the first act of The Haunted being divided into actual scenes. Thus, Mourning Becomes Electra is extraordinarily lengthy. In many productions, the length is cut for the sake of practicality, and the chorus of townsfolk cut from productions due to the expense, leaving only the eight main players.
