That was April 5, 2013.
On April 6, 2014, I had my next chance to look at him, this time as a member of the Houston Astros franchise, as he took the mound for the Lancaster Jethawks against the Lake Elsinore Storm.
It was an ideal day for the first-overall draft pick to show off his wares - an early-season Sunday matinee, with the famous Lancaster winds mostly still, except for occasional light breezes coming in from center field.
A week earlier, Appel had pitched for the first time in Minute Maid Park, and had wowed not just Dan Brooks, but also - perhaps more importantly - our own David Coleman.
So it was was with great anticipation that I arrived at The Hangar to see Appel's first full-season start, along a paid attendance of 2,312 ardent Jethawks supporters.
Appel strode to the mound and stared down the game's first batter, Alberth Martinez, who'd been absolutely crushing Jethawks pitching the entire series. His first pitch missed low, but he battled back to take a 1-2 lead before burying an offspeed pitch in the bottom of the zone, which the umpire called a ball, much to the home crowd's discontent.
Martinez would ground out to shortstop, but his plate appearance would become a trend: Not once during the game did a Lake Elsinore hitter swing at a Mark Appel pitch and miss it.
The following batter, Casey McElroy, would have flied out to center if not for a pretty ugly error by Teoscar Hernandez, inhabiting the same center field that in the past two years has held Andrew Aplin and George Springer, which allowed McElroy to reach third, where he would be chased in by a sacrifice fly.
The following inning began with a Corey Adamson triple. In fact, at times it seemed easier to count the number of batters Appel had faced without a runner on third base.
So what went wrong?
A few things, beginning with the sense that Appel never quite looked to be in any sort of a rhythm. His pitches had movement - even very good movement, in some cases - but very little snap. His fastball didn't sit heavy in the zone, rather leaking out the sides of the zone or even slipping through the bottom of it.
You know what? Instead of telling you, why don't I just show you instead?
Appel would escape the outing with limited damage, considering the lack of overwhelming stuff. His final line: 2.2 IP, 3 H, 2 R, 1 ER, 2 BB, 1 K. Mike Hauschild, in the second half of the piggyback, would come on and pitch masterfully to allow the Jethawks offense to scrape together a win, highlighted by a three-run moon shot to left-center field by Carlos Correa, and Brandon Meredith's first hit - and indeed his first home run - of the young season.
All in all, an inauspicious debut for the righty, but not as bad as the box score might suggest. And, as you're reading this, Appel is coming off of his second start of the season, which went much better, as he went a full five innings with six strikeouts, a single walk, three hits (a double shy of the cycle,) and two runs (both earned.) He induced five groundouts to just two flyouts. Certainly a better result than his first outing.