Agriculture technology is moving toward automation, placing operators in a supervisory role. This change in operator workload may lead to increased stress and higher mental load, resulting in reduced attention and hence greater risk of illness or injury to humans and damage to equipment. This study investigated the use of easily applicable equipment to measure mental load. Three methods were used to measure the mental load on machine operators: heart rate monitoring, two types of electroencephalograph (EEG) evaluation, and an assessment protocol. Three driving exercises (general driving, slalom driving, and loading) and a counting exercise were used in a driving simulator to create different levels of mental load. Due to the number of exercises, a single-scale assessment protocol was used to save time. We found that only the assessment protocol gave clear results and would work well as an evaluation tool. The heart rate and EEG measurements did not provide clear data for mental load assessment.