When most people think about job opportunities in public health, they tend to focus on government organizations such as the FDA, the CDC, the NIH, the WHO, or a state-level public health department. If you can demonstrate public health competency as a physician, then you won't need an MPH to get a public health job. Make sure you brush up on epidemiology and biostatistics.
The Association of Schools of Public Health (ASPH) has a public health job search engine at PublicHealthJobs.net.
Some physicians also forget about public health jobs in the private sector. Many jobs focus on population health management by including elements of disease management and patient education. Some organizations rely on the Internet to disseminate health information while others may use innovative approaches to impact population health.