In order to determine business mileage, a taxpayer must first determine their ‘Principal Place of Business’ (PPB). While not clearly defined by the IRS, the Supreme Court has defined PPB as the ‘nerve center’ (more information) of a taxpayer’s business.
Once the PPB is determined, then any mileage incurred with a personal vehicle used for business purposes is deductible from the point of the PPB. Business mileage must be supported by a log or mileage app such as Mile IQ (https://www.mileiq.com/).
For more information, please see IRS Pub 463 (link).