He credited one set of grandparents for “instilling a lot of Jewish traits in our family.” “I still love my religion,” said Horwitz. He played in the JCC Maccabi Games when he was 15 and 16, leading his LA-area team to national titles in 1996 and 1997.
In as a junior, he hit .347 with a team-leading 47 RBI and a .405 OBP and a .535 slugging percentage. He batted .400 with runners in scoring position, and was Pac-10 honorable mention and honorable mention Pac-10 All-Academic.
He was drafted in the 26th round by the Oakland Athletics in the June draft, but did not sign.
As a senior he dropped down to .288 and though he kept his OBP at .400, he wasn't drafted. "The two days of the draft were probably the worst two days of my life," he said. "I've kind of been doubted my whole career, and that's fine with me. It fuels the fire."
He had all the forms ready for chiropractic school, but he wanted the experience of playing pro ball for a half-summer, so he took the San Francisco Giants' offer of a uniform and $1,000 and signed as a non-drafted free agent in June of . "I used it to pay my phone bill," he said.
In 2004, Horwitz won the Northwest League batting title with a .347 average, the second-best in team history, playing for the Salem-Keizer Volcanoes in the Northwest League, and had a .407 OBP. He also earned a spot on the league All-Star team, led the league in hits (93), and set a franchise mark with 24 doubles.
In , he hit .349 for the Augusta GreenJackets in the South Atlantic League, and won his second consecutive batting title, while sporting a .415 OBP. He had 50 walks, while striking out only 39 times in 470 at-bats, and also led the league with 38 doubles. In addition, he had 8 outfield assists. He was also MVP of the 2005 California League Championship Series. Baseball America dubbed him the player with the best strike-zone discipline in the Giants' organization.
In , he spent nearly half the season at San Jose of the California League, where he hit .324 with a .414 OBP. He spent most of the rest of the season with the Double-A Connecticut Defenders, where he hit .286 with a .365 OBP, and ended the season with a handful of at-bats with the Triple-A Fresno Grizzlies.
In , he was a non-roster invitee to the Giants' spring training, but the Giants ultimately assigned him to the Connecticut Defenders. In 35 games for the team, he hit .309 with a .371 on base percentage. In late May, he was promoted to the Triple-A Fresno Grizzlies. He hit .326 for the Grizzlies with a .383 on base percentage in 84 games.
In his minor league career through 2007, Horwitz had a .326 batting average, .395 OBP, and in 1,630 at-bats has 109 doubles, 224 RBI, 168 walks, and 165 strikeouts.
Horwitz started with Fresno, and was hitting .294 with a .351 OBP in 44 games when he was called up to the major leagues.
Horwitz hit his first major league home run on June 2, 2008, off New York Mets starting pitcher Óliver Pérez. He was mobbed in the dugout afterward. "There was a lot of love in that dugout," Horwitz said. "It adds to the experience. That's a great group of guys here and I just love being a part of this team."
After hitting .304 in his first 21 at-bats, Horwitz went 1-for-15 and was sent down to Fresno for more playing time on July 6, 2008.