Will a cement mixer extend the working life of mortar?
A mixer might help you mix smaller batches more quickly, so that you can easily adjust the size of a batch and keep up. But most mixers will make you think you should be making an even larger batch, and that's not a great plan.
It won't perform magic, and trying to extend the working time of cement products is heading down a road that is better left untravelled. Far better to sort whatever you need to sort to make small batches accurately and make more of them.
Without knowing what method you were using, you may be better served by a proper mortar hoe that has two big holes in the blade to promote mixing than a mixer, since your problem was having too much mortar mixed up to get through. A beneficial side effect of smaller batches is that they are easier to hand-mix.
I recall being instructed to make sure the mixing pan and tools were cleaned between each batch, to prevent the partially set earlier batch from speeding up the set of the next batch.
If you just want to extend the working life, then chilling everything (water, sand, cement) will slow the reaction somewhat. Chilling only one part (water being easiest) will slow it somewhat less, as the rest of the materials will bring the temperature back up, and the reaction itself is exothermic. But mixing only the right-sized batch for your rate of work is easier. I use a scale that reads in grams when I mix tile mortar and grout, working by myself on fussy small tiles. I can't possibly get through a whole bag at a time within the working time, and I don't even try. –