Beginning in 1886, Mack played 10 seasons in the National League and one in the Players' League, for a total of 11 seasons in the major leagues, almost entirely as a catcher.

Beginning in 1884, he played on minor league teams in the Connecticut cities of Meriden and Hartford before being sold to the Washington Nationals (sometimes called the Statesmen or the Senators) of the National League in 1886.
In the winter of 1889, he jumped to the Buffalo Bisons of the new Players' League, investing his entire life savings of $500 in shares in the club.
But the Players' League went out of business after only a year, and Mack lost his job and his whole investment.
In December 1890 Mack signed a contract with the Pittsburgh Pirates of the National League and remained with them for the rest of his career as a full-time player.

As a player, Mack was "a light-hitting catcher with a reputation as a smart player, but didn't do anything particularly well as a player."

Mack was one of the first catchers to position himself directly behind home plate instead of in front of the backstop.
According to Wilbert Robinson, "Mack never was mean ... [but] if you had any soft spot, Connie would find it.
He could do and say things that got more under your skin than the cuss words used by other catchers."

In addition to verbally needling batters to distract them, he developed skills such as blocking the plate to prevent base runners from scoring and faking the sound of a foul tip. (
He was probably responsible for the 1891 rule change requiring that a batter must have two strikes against him in order to be called out if the catcher caught a foul tip.)
Besides tipping bats to fake the sound of a foul tip, Mack became adept at tipping bats to throw off the hitter's swing. ("
Tipping" a bat is to brush it with the catcher's mitt as the batter swings, either delaying the swing or putting it off course, so that the batter misses the ball or doesn't hit it solidly.
If the umpire is aware that a bat has been tipped, whether intentionally or unintentionally, he calls catcher's interference.)
Mack never denied such tricks:
Farmer Weaver was a catcher-outfielder for Louisville.
I tipped his bat several times when he had two strikes on him one year, and each time the umpire called him out.
He got even, though.
One time there were two strikes on him and he swung as the pitch was coming in.
But he didn't swing at the ball.
He swung right at my wrists.
Sometimes I think I can still feel the pain.
I'll tell you I didn't tip his bat again.
No, sir, not until the last game of the season and Weaver was at bat for the last time.
When he had two strikes, I tipped his bat again and got away with it.
Beginning in 1886, Mack played 10 seasons in the National League and one in the Players' League, for a total of 11 seasons in the major leagues, almost entirely as a catcher.
Beginning in 1884, he played on minor league teams in the Connecticut cities of Meriden and Hartford before being sold to the Washington Nationals (sometimes called the Statesmen or the Senators) of the National League in 1886. In the winter of 1889, he jumped to the Buffalo Bisons of the new Players' League, investing his entire life savings of $500 in shares in the club. But the Players' League went out of business after only a year, and Mack lost his job and his whole investment. In December 1890 Mack signed a contract with the Pittsburgh Pirates of the National League and remained with them for the rest of his career as a full-time player.
As a player, Mack was "a light-hitting catcher with a reputation as a smart player, but didn't do anything particularly well as a player."
Mack was one of the first catchers to position himself directly behind home plate instead of in front of the backstop. According to Wilbert Robinson, "Mack never was mean ... [but] if you had any soft spot, Connie would find it. He could do and say things that got more under your skin than the cuss words used by other catchers."
In addition to verbally needling batters to distract them, he developed skills such as blocking the plate to prevent base runners from scoring and faking the sound of a foul tip. (He was probably responsible for the 1891 rule change requiring that a batter must have two strikes against him in order to be called out if the catcher caught a foul tip.) Besides tipping bats to fake the sound of a foul tip, Mack became adept at tipping bats to throw off the hitter's swing. ("Tipping" a bat is to brush it with the catcher's mitt as the batter swings, either delaying the swing or putting it off course, so that the batter misses the ball or doesn't hit it solidly. If the umpire is aware that a bat has been tipped, whether intentionally or unintentionally, he calls catcher's interference.) Mack never denied such tricks:
Farmer Weaver was a catcher-outfielder for Louisville. I tipped his bat several times when he had two strikes on him one year, and each time the umpire called him out. He got even, though. One time there were two strikes on him and he swung as the pitch was coming in. But he didn't swing at the ball. He swung right at my wrists. Sometimes I think I can still feel the pain. I'll tell you I didn't tip his bat again. No, sir, not until the last game of the season and Weaver was at bat for the last time. When he had two strikes, I tipped his bat again and got away with it.
Connie Mack