It depends on the form of lowball poker played. Most forms of lowball poker I'm familiar with classify the hand first, then rank the cards from lowest to highest.
In your example, assuming that Aces count low:
As 4h 8d Jc Qs and Ah 2d 5c 9s Kc are both high card hands (no pairs or better). Thus, we compare the lowest cards until we find a mismatch.
Both hands have an Ace. Next lowest card is the 4h and the 2d, so Ah 2d 5c 9s Kc would be the low hand.
If the second hand was Ah 2d 2c 9s Kc, As 4h 8d Jc Qs would be considered lower because it's an unpaired hand, which is ranked lower than a paired hand.
Note that lowball poker variants can significantly differ with what counts for a low hand:
In 2-7 lowball poker, Aces are considered high only, and straights and flushes count for qualifying a hand (so a hand with 23456 would be considered higher than 23457 because the former hand makes a straight).
However, in Razz, a form of lowball stud poker, Aces are low and straights and flushes don't count against your hand for a low (so A2345, all the same suit, is the lowest hand you can have).
In split pot games, where the pot is split in half between the best low hand and the best high hand, there is often a qualifying condition for your low hand to be valid. The most common one is "8-or-better"; the 5 cards making the low card need to be all rank 8 or lower.
