Dannelley, 24, was born 30 miles northwest of downtown Houston in Tomball, Texas. At Comesneil High School, he excelled as a two-way player, but he truly thrived on the mound. He finished his prep career ranked in the top five in the state in ERA, WHIP, and strikeouts. 

His college journey began at Panola College in Carthage, Texas, in 2020. By the following year, he had been named captain of the baseball team. Dannelley remained in Texas when he transferred to the University of Houston, where he pitched in 18 games during his lone season with the Cougars, posting a 5.21 ERA and recording one save. 

In the summer of 2022, Dannelley headed to Lafayette, Indiana, to pitch for the Lafayette Aviators of the Prospect League. After the season, he chose to stay in the area and transferred to Purdue University. The right-hander graduated after two years as a Boilermaker, appearing in 41 games and striking out 74 batters across 73 innings. 

Following graduation, Dannelley joined the Morehead City Marlins of the Coastal Plain League. He pitched in eight games—three of them starts—recording a 1.89 ERA and striking out 25 hitters in 19 innings. The Chicago Dogs signed him to his first professional contract that August, and he appeared in five games, finishing with a 10.57 ERA. 

Dannelley then traveled to Charlotte, North Carolina, to train at Tread Athletics. During a session in front of scouts, the Atlanta Braves agreed to sign him. He spent the 2025 season in A-ball with the Augusta GreenJackets, making 30 relief appearances. He posted a 5.51 ERA, striking out 48 and walking 26 across 50.2 innings.