On the remote frontier they were fairly common but in more settled areas, especially with family nearby there usually was a regular marriage. Often times in the Old West, couples, well…coupled, without a license or ceremony because there was no preacher or judge around to marry them. I would guess in the more informal West a couple might also have a common law divorce. They just went their separate ways. Sometimes the couple couldn’t afford to pay the preacher, rabbi or the priest.
The society of sporting men, gamblers had a long list of women who used their lover’s last name. In todays lingo they “hooked up.”
As far as common law divorce today, you can’t get out of a common law marriage as easy as you got into it. There might not be a marriage contract but everything else is a contract and a couple must petition the appropriate court in order to dissolve their marriage
Wyatt Earp is a good example of how the West made its own rules when it came to such matters. Urilla Sutherland was his first wife, legally, but she died early in the marriage. Wyatt had other women who used his last name. One, Sally Haspel, was a teenage prostitute working in her mother’s brothel. Mattie Blaylock, also a prostitute, was living with him as his common law wife in Tombstone, but they parted ways without a divorce and he took up with another “sporting” lady, Sadie Marcus. There’s no record of their marriage either.
Common law marriages were valid under English Common Law. In the 1877 case Meister v. Moore, the U.S. Supreme Court held that a non-ceremonial marriage was a valid enforceable marriage, unless a state’s statute forbade it. Common law marriages differ from state to state.