William Earle's main business on his return to Liverpool in July 1752 was of a romantic nature, his courtship of Mrs Anne Winstanley, whom he married a year later. Anne was a widow and, although no doubt much loved by her ‘faithfull admirer’, it seems likely that her attractions included a substantial dowry to add to what William had been able to save from his five years as a ship's captain. These savings, too, would probably have been substantial, as being captain of a slave ship was well known as a path to comparative riches for a young man in Liverpool.

The pay of all ships’ captains was reasonably good, from £5 to £8 a month all found, but slave ship captains were able to accumulate far more, some of this extra remuneration being spelled out in their instructions. William was ‘to have for your privilege five slaves’, such slaves being hand-picked by the captain, fed at the ship's expense and often trained up by the captain or one of his crew to provide them skills that would inflate their value on arrival in the West Indies. And it was said that the captain's privilege slaves never died, being quietly replaced by healthy ones if necessary. Captains also received a commission on the cargo and many of them dabbled in private trade as well, shipping gold dust or ivory on their own account. Such rewards and earnings could add up to nearly £1,000 for a single voyage.

Such accumulation was obviously invaluable, indeed essential, for the next stage of William's career in the slave trade, as a merchant and shipowner. Nearly all slave merchants owned shares in ships and William's shipowning interests can be pieced together from the Slave Trade Database. This shows that William Earle senior was either a captain or an investor in exactly 100 voyages. Three of these were the voyages already discussed, in which William had command but no share of ownership, leaving a total of 97 voyages between 1753 and 1787 in which William was a part-owner, making him the tenth most important slave merchant in Liverpool.